In electronic pre-press systems or for desktop publishing, images are usually offered in electronic form, and are then referred to as electronic images. These electronic images can be obtained by scanning photographic images by an electronic image scanner or by capturing a scene in an electronic camera. These electronic images can be stored permanently on magnetic disk, which can be inserted in a layout system. Alternatively, the electronic images can be transported via direct links or networks to the layout system. On the layout system, one or more electronic images or portions thereof can be inserted interactively in a layout for a page, for printing a reproduction of the electronic image. The creation of a page layout results in an electronic data stream or electronic file describing the several elements of the page layout in electronic format. This electronic page layout is usually expressed in a page description language such as POSTSCRIPT (trademark of Adobe Systems Incorporated). AGFASCRIPT (trademark of Agfa-Gevaert A.G., Leverkusen in Germany) etc. The electronic page layout comprises the data for each electronic image that must appear on the printed reproduction. Usually the electronic images are continuous tone, which is imagery containing multiple grey levels with no perceptible quantisation to them. These continuous tone or contone images can be black and white or colour images. An electronic colour image is separated in a set of colour components. Each electronic colour component is equivalent with a black and white contone electronic image. Printing each colour component on top of each other, using the appropriate coloured ink for that colour component, gives a reproduction of the electronic colour image. The separation process of an electronic colour image into electronic colour components is a transformation achieved by a colour management system, which must know about the characteristics of the different coloured inks and makes some assumptions about the relation between grey levels and densities. A printing device must thus be capable to accept one or more electronic colour components and render it on hardcopy using the appropriate ink. This ink is black for black and white image reproductions, and is usually cyan, magenta, yellow and black for the different colour components of colour images. Such a printing device converts each electronic grey level, which is usually an eight bit number having a value N between 0 and 255, to a visual density on the hard copy. This density D is defined as the negative base ten logarithm of the portion of reflected or transmitted light from incident light on the hard copy, and can be measured by a densitometer. The relation between the grey level N and the density D is strongly dependent on the output device. In order to get the same reproduction from the same electronic page layout on different output devices, not the grey level N is fed directly to the printing device, but a stimulus value S is applied to the printing device. The stimulus value S is derived from the grey level N in such a manner that after transformation of the stimulus S to a density D by the printing device, a specific relation exists between the grey level N and the resulting density D. This is called linearization of the printing device, and is thus achieved by establishing a device specific relation between the grey level N and the device stimulus S. This relation is referred to as a tone curve or a transfer function. It is known in the art that most printing devices have a device specific transfer function incorporated, obtained by calibration. Each time when electronic contone images are printed on this device, the same device specific transfer function is applied to the grey levels of the electronic image. This linearization is also a necessary condition for the proper behaviour of the colour management system.
Problems arise when the output mode of the output device changes. One output device can for example render electronic images on transparent photographic film or on photographic paper or on another output medium. The term medium or media indicates the physical material on which the output appears (paper, transparency material, film, or whatever). It is also possible that the sensitometric properties of the photographic emulsions are different for different types of film. In such a case, an operator has to calibrate the printing device again for establishing another transfer function and install this transfer function in the device. This way of working is very problematic when different output media can easily be exchanged.
In electronic pre-press systems, the output mode of the output device can even be influenced by other factors. Such an output device has to produce a photographic output with halftone pictorial, which is composed ideally of only two grey levels: black and white. In order to reproduce continuous tone imagery, the illusion of continuous tone for the human visual system is achieved by halftoning or screening. A whole technology of screening techniques has been developed over several years, an overview of which can be found in "Evaluation of Clustered-Dot Halftoning Technology in the American Patent Literature" by Peter R. Jones and in "A Survey of Electronic Techniques for Pictorial Image Reproduction" by J. C. Stoffel et al., published in IEEE Transactions on Communications, vol. COM29, no. 12, December 1981 on pages 1898-1925. On one single graphical recorder or output device, a choice of several screening techniques can be offered, such as autotypical screening or frequency modulated halftone screening. When using different types of screening on the same output device, even with the same output media, differences in density reproduction are obtained. Even the line ruling or screening angle in autotypical rasters can influence the density reproduction. This poses not only problems for the colour management system, but also for the same electronic page layout, the outcome of which strongly depends on the screening parameters, which are usually defined in one of the last stages of the electronic pre-press workflow. The problem becomes even more difficult to solve or very conspicuous when it is not properly solved in the case where two electronic images to be rendered on the same page must be screened by different screening techniques. This is true for as well black and white images as for colour images. EP-A-0 334 518, EP-A-0 538 901, EP-A-0 533 593, GB-A-2,174,265 and U.S. Pat. No. 4,516,135 disclose analoguous techniques for making use of tone transformation.